Introduction (week 1+) - GitHub Pages



introduction (week 1+)Ben Bolker03 September 2019IntroductionAdministrative triviaInstructors: Dr.?Benjamin Bolker and Dr.?Weijie PangTAs: Nik Po?u?a, Steve Cygu, Aghigh Farhadi (marking)course web page: outline: (10%)Quizzes (5%)Final project (5%)Midterm tests (2 × 20%)Final exam (40%)homework assignment announcements policy(web page, Avenue: not in class)Laptop policyCourse material on web page and Avenue to LearnExpectations of professor and studentsTextbook (optional); Gries et al. Practical Programming 3d ed. (see outline)also see resourcesCourse contentreasonable balance amongnitty-gritty practical programming instruction:… I just sat down in front of a text editor, with nothing but thoughts, and ended up with a program that did exactly what I wanted it to a few hours later … (ankit panda)conceptual foundations of computing/computer sciencecontext/culture of mathematical/scientific computinginteresting applicationsInstalling PythonCodeLab: must have access to a computer with Python3 installed.See installation instructionsOverview of math/sci computingUsing computers in math and sciencemath users vs.?understanders vs.?developersdevelop conjectures; draw pictures; write manuscriptsmathematical proof (e.g. four-colo(u)r theorem and other examples); computer algebraapplied math: cryptography, tomography, logistics, finance, fluid dynamics, …applied statistics: bioinformatics, Big Data/analytics, …discrete vs.?continuous mathRunning Pythonvia notebooks ( or on your own computer)via scripts + console ()Fun!Hello, world (always the first program you write in a new computer language)print('hello, python world!')## hello, python world!Python as a fancy calculator (REPL, Read-Evaluate-Print-Loop)print(62**2*27/5+3)## 20760.6reference: Python intro section 3.1.1Interlude: about Pythonprogramming languagesPython: scripting; high-level; glue; general-purpose; flexiblecontrast: domain-specific scripting languages (MATLAB, R, Mathematica, Maple)contrast: general-purpose scripting languages (Perl, PHP)contrast: general-purpose compiled languages (Java, C, C++) (“close to the metal”)relatively modern (1990s; Python 3, 2008)currently the 5th most popular computer language overall (up from 8th in 2015); most popular for teachingwell suited to mathematical/scientific/technical (NumPy; SciPy; Python in Finance)ex.: Sage; BioPythonthe “prime walk” (from math.stackexchange.ca)start at the origin, heading right, counting up from 1move forward one space, counting up, until you find a primeturn 90° clockwiserepeat steps 2 and 3 until you get boredcode here (bbolker.github.io/math1mp/code/primewalk.py)Note:easier to understand/modify than write from scratchbuild on existing components (modules)Interfacesintegrated development environment (IDE), command line/console (Spyder)programming editornotebooksnot MS Word! Featuressyntax highlighting, bracket-matching, hot-pastingintegrated helpintegrated debugging toolsintegrated project management toolsmost important: maintain reproducibility; well-defined workflowsAssignment and types (PP $\S2.4$)superficially simpleset aside memory space, create a symbol that points to that space= is the assignment operator (“gets”, not “equals”)<variable> = <value>variable nameswhat is legal? (names include letters, numbers, underscores, must start with a letter)what is customary? convention is variables_like_this (“snake case”)what works well? v vs. temporary_variable_for_loopsame principles apply to file, directory/folder namesvariables are of different typesbuilt-in: integer (int), floating-point (float), complex, Boolean (bool: True or False),dynamic typingPython usually “does what you mean”, converts types when sensiblestrong typingtry print(type(x)) for different possibilities (x=3; x=3.0; x="a")what happens if you try x=a?don’t be afraid to experiment!Examplesx=3y=3.0z="a"q=complex(1,2)type(x+y) ## mixed arithmetictype(int(x+y)) ## int(), float() convert explicitlytype(x+z)type(q)type(x+q)type(True)type(True+1) ## WAT[^2](As Dive into Python says in a similar context, “Ew, ew, ew! Don’t do that. Forget I even mentioned it.”)Check out the Python tutor for these examplesArithmetic operators, precedenceexponentiation (**)negation (“unary minus”) (-)multiplication/division (*,/,//=integer division,%=remainder (“modulo”))addition/subtraction (+, - (“binary”))Use parentheses when in doubt!Puzzle: what is -1**2? Why?Logical operators (PP $\S5.1$)comparison: (==, !=)inequalities: >, <, >=, <=,basic logic: (and, or, not)remember your truth tables, e.g. not(a and b) equals (not a) or (not b)a = True; b = False; c=1; d=0a and bnot(a and not b)a and not(b>c)a==c ## careful!not(d)not(c)operator precedenceremember order of operations in arithmeticnot has higher precedence than and, or. When in doubt use parentheses …From CodingBat:We have two monkeys, a and b, and the parameters a_smile and b_smile indicate if each is smiling. We are in trouble if they are both smiling or if neither of them is smiling. Return True if we are in trouble.monkey_trouble(True, True) → Truemonkey_trouble(False, False) → Truemonkey_trouble(True, False) → FalseTruth tablesABA and BA or Bnot ATrueTrueTrueTrueFalseTrueFalseFalseTrueFalseFalseTrueFalseTrueTrueFalseFalseFalseFalseTrueLogical expressionsThe logical expression: not not a and not b or a is equivalent to ((not (not a)) and (not b)) or a since the operator not takes precedence over the operators and and or.So if a = True and b = False this evaluates to TrueSince not not a is equivalent to a, we can simplify the expression to just (a and not b) or a.Can we simplify this further?What can we do with not a and not b ?More CodingBat problemssquirrel_playcigar_partyString operations (PP chapter 4)reference: Python intro section 3.1.2Less generally important, but fun+ concatenates* replicates and concatenatesin searches for a substringa = "xyz"b = "abc"a+1 ## errora+bb*3(a+" ")*5b in aCodingBat problems:make_abbamake_tagsOne more useful string operation: len(s) returns the length (number of characters)Indexing and slicingIndexingExtracting elements is called indexing a listIndexing starts from zeroNegative indices count backward from the end of the string(-1 is the last element)Indexing a non-existent element gives an errorslicingSlicingExtracting (consecutive) sets of elements is called slicingSlicing non-existent element(s) gives a truncated resultSlicing specifies start, end, step (or “stride”)Leaving out a bit goes from the beginning/to the endSlicing works on strings too!x[:] # everythingx[a:b] # element a (zero-indexed) to b-1x[a:] # a to endx[:b] # beginning to b-1x[a:b:n] # from a to b-1 in steps of ngenerate a list of odd numbers from 3 to 15reverse a string?String slicing practiceFrom CodingBat:first_twofirst_halfMethodsObjects in Python have classes (string, integer, etc.)Classes have methods - things you can to do the objectsYou use a method by calling .()yes, this seems weird at first.methods may or may not have argumentsString methods: examplesStrings have lots of methods, for example:x = "abcdef"x.upper()## 'ABCDEF'x.capitalize()## 'Abcdef'x.endswith("f")## Truex.startswith("qrs")## Falsex.islower()## True ................
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